Transcript for Royal Family Portrait: Will and Kate Show the World Their New Baby Boy

There was a worldwide sensation today over a one-day old baby. This was the scene. A tiny prince emerging from the hospital.Nts in casual clothes and later when they departed, dad sporting a car seat. Somehow all of us todayad a sense that history is finally moving forward. And abc's amy robach is in london reporting on it tonight. Reporter: It was the moment that everyone has been waiting so many weeks for. Last night the royal family, just the three of them, spent the night in the hospital, saint mary's here behind me, and then walked down those now very famous steps and presented their son to the world. Proud parents william and kate with their day-old son. The future king's first royal wave on the steps of the hospital. Well, he's got a good pair of lungs on him, that's for sure. He's a big boy. He's quite heavy. We're still working on the name. Reporter: In a carefully choreographed hand-off, william holds his baby boy. Look closely, one lip reader telling us kate told him, got him. Watch his head. Very emotional. Yes. Very emotional. It's such a special time. I think any parent, I think, will probably will sort of know what this feeling is like. Very special. Reporter: The couple at ease with their new son, and each other. His hand resting lightly on her back. He's got her looks, thankfully. No, no. Even joking about baby cambridge's head of hair. He's got way more than me, thank god. Reporter: A different scene from william's debut 31 years ago. Mother and father standing formally apart. So different from the relaxed couple today. William's shirt even unbuttoned at the collar. Kate also wearing poke adots. The proud grandparents on both sides visiting today. He's absolutely beautiful. They're both doing really well. We're so thrilled. Reporter: It will be a modern upbringing for this newest royal and a balancing act. William, raised surrounded by cameras and surrounded by tradition. Blended with kate's down to earth upbringing. Tonight, the couple looking like new parents everywhere. The prince carrying his son to the car. Clipping in his car seat before driving his new family home. And amy, I want to go back to that car seat for a moment. Who hasn't wrestled with one of those. We saw him do it. We put a clock on it. He got it all done in about 8 seconds as we watched and I know you were just a few feet away. What were you seeing? I have to say I personally struggle with those car seats before. I was utterly impressed. I think he was both proud and relieved when he clicked it right in. Of note, diane, that car seat, a very affordable choice for the royal couple, $179. I think it's safe to say it will probably sell out tomorrow. Amy robach reporting in from london. Thank you.

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